Field of the Invention
The present invention relates to a magnetic recording medium manufacturing method, a magnetic recording medium manufacturing apparatus and a magnetic recording medium.
Description of the Related Art
A magnetic recording medium such as a magnetic tape may include the following two types: a conventional application-type tape fabricated by applying magnetic coating material, which is made by dispersing magnetic powder in a binder, to a film serving as a non-magnetic support; the other is a deposition-type tape fabricated by depositing metal on a film without any binder, in a vacuum.
The deposition-type tape is desirable for high-density recording because it does not contain binder in a magnetic layer, so that the density of the magnetic material can be increased.
FIG. 3 shows the main constitution of a deposition-type tape that is currently commercially available.
A non-magnetic support 51, e.g., a film, is made of PET (polyethylene terephthalate), polyimide or aramid. The film thickness is 2 to 50 .mu.m.
A magnetic layer 52 is formed by depositing Co-Ni alloy (80%-20%) on the non-magnetic support 51 by a vacuum deposition method to have a thickness of 1,500 .ANG.. An undercoat layer optionally may be formed between the non-magnetic support 51 and the magnetic layer 52 in order to improve the adhesive property of the magnetic layer 52.
A top-coat layer 53 is formed to protect the magnetic layer 52 and serve as a lubricant for the smooth contact with a recording reproducing head. The coating method may be either a gravure technique or a die-coating technique. Perfluoropolyether (e.g., FOMBLIN (trade name) produced by Montedison S.p.A.), which is a fluorine-containing lubricant, is used as the component for the top-coat layer.
A back-coat layer 54 is made of a coating material prepared by dispersing carbon black (grain diameter of 10 to 100 nm) in a binder (chloroethylene-, urethane-, or nitrocellulose-based binder alone or a mixture of them). The coating material is applied to the surface opposite to the deposition surface of the magnetic layer 52 of the non-magnetic support 51 by a gravure technique, a reverse technique or a die-coating technique, so that the thickness of carbon after drying is 0.4 to 1.0 .mu.m.
The back-coat layer mainly has the following functions.
(1) To prevent dust from adhering to the tape because of the antistatic effect obtained by imparting an electrical conductivity to the layer.
(2) To stabilize the tape transport by controlling the surface property (friction coefficient).
(3) To prevent warpage by balancing the front and back of the magnetic layer in view of hardness and so on.
Conventionally, the back-coat layer is formed by applying a coating material which is made by dispersing carbon black in a binder. Function (1) is met by the conductivity of the carbon black; function (2) is met by controlling carbon black grain diameter and the method of application; and function (3) is met by controlling the binder and the application thickness.
However, the back-coat layer of a conventional, deposition-type tape is of the application type. Therefore, when applying the back-coat layer before vacuum-depositing the magnetic layer, degassing (gas generated from residual solvent in the coated layer from the back-coat layer) occurs in the vacuum system and the degree of vacuum is lowered. As a result, the deposition is not well performed.
Therefore, conventionally, after the magnetic layer is deposited in a vacuum, the tape is taken out into the air and the back-coat layer is coated.
However, this method involves a problem with air borne dusts adhering to the film and contaminating the magnetic layer in the step of applying the back-coat layer, increasing the number of dropouts found in the dropout inspection (inspection for detecting dropouts, which are dropouts of reproduced signals due to scratches of the tape surface or foreign matter adhering to the surface, wherein the magnetic tape is loaded in an inspection cassette deck and predetermined signals are recorded and reproduced simultaneously).
Moreover, the conventional method causes a problem, in that the electrical conductivity of the back-coat layer is lowered, and consequently the antistatic effect thereof is marred, because carbon is mixed with an organic binder.
The electric conductivity, as a resistance value (surface resistance), below 10.sup.7 .OMEGA. is applicable, and the value for a marketed 8-mm video tape is 10.sup.6 .OMEGA.. However, a higher conductivity (lower resistance value) is demanded for a back-coat layer using carbon black. The reason for this is that the electric conductivity of the back-coat layer is lower than that of a back-coat layer using a metal or semi-metal, because the carbon black must be provided with an organic binder, thereby lowering conductivity, while the metal or semi-metal is provided alone, without any organic binder.
As described above, although a fluorine-containing lubricant is frequently used for the top-coat layer of a metallic-thin-film-type magnetic recording medium, the fluorine-containing lubricant is partially decomposed at 200.degree. C., and its vapor pressure is low. As a result, it is impossible to apply the fluorine-containing lubricant in a vacuum. Thus, the method of preparation of the top-coat layer is an important factor with respect to the productivity of the magnetic recording medium, because it requires separate coating line in the air.
JP-A 5431708 discloses that a magnetic recording medium can be produced by depositing a non-magnetic layer and a ferromagnetic layer on a substrate in vacuum, while the partial pressure of oxygen gas is being controlled. The non-magnetic layer is formed from Cr, Ti, Si or Sn. However, the "nonmagnetic layer" is an undercoat layer applied as an adhesive between the magnetic layer and the nonmagnetic substrate. This publication does not disclose a back-coat layer to be applied to the other surface of the nonmagnetic substrate.
JP-A 54-141111 discloses that a magnetic recording medium is produced by directing magnetic vapor, containing accelerated ions, onto a substrate passing through a cylindrical can in a vacuum. A suitable degree of vacuum is obtained by introducing oxygen gas from outside the can.